This application relates to improvements in hybrid vehicles, that is, vehicles in which both an internal combustion engine and one or more electric motors are provided to supply torque to the driving wheels of the vehicle, and wherein turbocharging is employed under certain circumstances. A preferred method of sizing the power-producing components of the hybrid vehicle is also disclosed.
This application discloses a number of improvements over and enhancements to the hybrid vehicles disclosed in U.S. Pat. No. 5,343,970 (the xe2x80x9c""970 patentxe2x80x9d) to one of the present inventors, which is incorporated herein by this reference. Where differences are not mentioned, it is to be understood that the specifics of the vehicle design shown in the ""970 patent are applicable to the vehicles shown herein as well. Discussion of the ""970 patent herein is not to be construed to limit the scope of its claims.
Generally speaking, the ""970 patent discloses hybrid vehicles wherein a controllable torque transfer unit is provided capable of transferring torque between an internal combustion engine, an electric motor, and the drive wheels of the vehicle. The direction of torque transfer is controlled by a microprocessor responsive to the mode of operation of the vehicle, to provide highly efficient operation over a wide variety of operating conditions, and while providing good performance.
The flow of energyxe2x80x94either electrical energy stored in a substantial battery bank, or chemical energy stored as combustible fuelxe2x80x94is similarly controlled by the microprocessor. For example, in low-speed city driving, the electric motor provides all torque needed responsive to energy flowing from the battery. In high-speed highway driving, where the internal-combustion engine can be operated efficiently, it typically provides all torque; additional torque may be provided by the electric motor as needed for acceleration, hill-climbing, or passing. The electric motor is also used to start the internal-combustion engine, and can be operated as a generator by appropriate connection of its windings by a solid-state, microprocessor-controlled inverter. For example, when the state of charge of the battery bank is relatively depleted, e.g., after a lengthy period of battery-only operation in city traffic, the internal combustion engine is started and drives the motor at between 50 and 100% of its maximum torque output, for efficient charging of the battery bank. Similarly, during braking or hill descent, the kinetic energy of the vehicle can be turned into stored electrical energy by regenerative braking.
The hybrid drive train shown in the ""970 patent has many advantages with respect to the prior art which are retained by the present invention. For example, the electric drive motor is selected to be of relatively high power, specifically, equal to or greater than that of the internal combustion engine, and to have high torque output characteristics at low speeds; this allows the conventional multi-speed vehicle transmission to be eliminated. As compared to the prior art, the battery bank, motor/generator, and associated power circuitry are operated at relatively high voltage and relatively low current, reducing losses due to resistive heating and simplifying component selection and connection.
Application Ser. No. 09/264,817 filed Mar. 9, 1999 (the xe2x80x9c""817 applicationxe2x80x9d), now U.S. Pat. No. 6,209,672, also incorporated herein by reference, discloses and claims certain further improvements in hybrid vehicles, described below, with respect to the vehicles of the ""970 patent. The present patent application represents further improvements over the hybrid vehicle described in the ""817 application.
It is an object of the present invention to provide further improvements over the hybrid vehicles shown in the ""970 patent and the ""817 application.
It is a more specific object of the present invention to provide a hybrid drive system for vehicles that does not require the controllable torque-transfer unit shown in the ""970 patent, while providing the functional advantages of the hybrid vehicles shown in the ""970 patent and the ""817 application, together with further improvements.
Other aspects of and improvements provided by the present invention will appear below.
According to the invention of the ""817 application, the controllable torque-transfer unit shown in the ""970 patent is eliminated by replacing the single electric motor shown therein by two separate motors, both operable as generators when appropriate, connected by a functionally-conventional clutch or mechanical interlock operated by the microprocessor responsive to the vehicle""s mode of operation and to input commands provided by the operator of the vehicle. As in the ""970 patent, an internal combustion engine is provided, sized to provide sufficient torque for the maximum cruising speed desired without requiring a multi-speed transmission, and is used for battery charging as needed. According to the invention of the ""817 application, a relatively high-powered xe2x80x9ctractionxe2x80x9d motor is connected directly to the output shaft of the vehicle; the traction motor provides torque to propel the vehicle in low-speed situations, and provides additional torque when required, e.g., for acceleration, passing, or hill-climbing during high-speed driving. A relatively low-powered starting motor is also provided, and can be used to provide torque propelling the vehicle when needed. This second motor is connected directly to the internal combustion engine for starting the engine. Unlike a conventional starter motor, which rotates an internal combustion engine at low speed (60-100 rpm) for starting, necessitating provision of a rich fuel/air mixture for starting, the starter motor according to the invention spins the engine at relatively high speed (typically 300 rpm) for starting; this allows starting the engine with a near-stoichiometric mixture, significantly reducing undesirable emissions and improving fuel economy at start-up.
As noted, the two motors are separated by a functionally-conventional clutch, that is, a clutch which either joins the two motors together for rotation at the same speed, or separates them completely. As the motor shafts can be controlled to rotate at essentially the same speed when the clutch is engaged, the clutch need not allow for significant slipping before engagement. Accordingly, a friction clutch, as normally provided for road vehicles, may not be required, and a less-expensive simple mechanical interlock may alternatively be employed. Engagement of the clutch is controlled by the microprocessor, e.g., controlling a hydraulic actuator, responsive to the state of operation of the vehicle and the current operator input.
For example, during low-speed operation, the clutch will be disengaged, so that the traction motor is disconnected from the engine; the vehicle is then operated as a simple electric car, i.e., power is drawn from the battery bank and supplied to the traction motor. If the batteries become depleted, the starter motor is used to start the internal combustion engine, which then runs at relatively high torque output (e.g., between about 50-100% of its maximum torque), for efficient use of fuel, and the starting motor is operated as a high-output generator to recharge the battery bank. If the operator calls for more power than available from the traction motor alone, e.g., in accelerating onto a highway, the starter motor starts the internal combustion engine, and the clutch is engaged, so that the engine and starter motor can provide additional torque. The engine is sized so that it provides sufficient power to maintain a suitable highway cruising speed while being operated in a torque range providing good fuel efficiency; if additional power is then needed, e.g., for hill-climbing or passing, the traction and/or starter motors can be engaged as needed. Both motors can be operated as generators, e.g., to transform the vehicle""s kinetic energy ito electrical power during descent or deceleration.
It is also within the scope of the invention to provide power from the engine and starting motor to one pair of road wheels, through the clutch, and from the traction motor to another set of wheels; this provides all-wheel drive, when needed, without a transfer gearbox or drive shaft. See provisional patent application Ser. No. 60/122,478, filed Mar. 1, 1999, incorporated by reference herein. In a further embodiment, torque from the engine, starter motor, and a first traction motor can be provided to a first set of road wheels, and torque from a second traction motor to a second set of road wheels; this would provide maximal flexibility in control of the transfer of torque to the road, useful in low-traction conditions.
In each of these aspects of the operation of the vehicle, and as in the ""970 patent, the operator of the vehicle need not consider the hybrid nature of the vehicle during its operation, but simply provides control inputs by operation of the accelerator and brake pedals. The microprocessor determines the proper state of operation of the vehicle based on these and other inputs and controls the various components of the hybrid drive train accordingly.
According to the present invention, the engine is further provided with a turbocharger, also controlled by the microprocessor, and operated only under extended high-load conditions. In low-speed driving, the turbocharger is bypassed and is inactive, so that the vehicle is operated as in the ""817 application; similarly, when the torque provided by the engine is inadequate for short-term high loads, such as during overtaking on the highway, the traction motor is employed to propel the vehicle. The starting motor may also be employed to provide torque as needed. However, according to the present invention, when conditions demand production of high torque for extended periods, for example, when towing a trailer, climbing a long hill, or driving at sustained high speed, or when the battery bank is relatively discharged, the microprocessor activates the turbocharger, so that additional torque is produced by the internal combustion engine when needed.
More specifically, in the ""817 application, during substantially steady-state operation, e.g., during highway cruising, the control system operates the engine at varying torque output levels, responsive to the operator""s commands. The range of permissible engine torque output levels is constrained to the range in which the engine provides good fuel efficiency. Where the vehicle""s torque requirements exceed the engine""s maximum efficient torque output, e.g., during passing or hill-climbing, one or both of the electric motors are energized to provide additional torque; where the vehicle""s torque requirements are less than the minimum torque efficiently provided by the engine, e.g., during coasting, on downhills or during braking, the excess engine torque is used to charge the batteries. Regenerative charging may be performed simultaneously, as torque from the engine and the vehicle""s kinetic energy both drive either or both motors in generator mode. The rate of change of torque output by the engine may be controlled in accordance with the batteries""state of charge.
According to the present invention, the above control strategy is retained and employed under substantially all xe2x80x9cnormalxe2x80x9d driving conditions; addition of a turbocharger controlled by the microprocessor according to the invention allows additional control flexibility. More specifically, when conditions demand power in excess of the engine""s normally-aspirated maximum output for a relatively long period of time, e.g., for climbing long hills, for towing, or when driving at high speed, the turbocharger, which is normally bypassed and thus inactive, is energized by supply of the engine""s exhaust gas stream. The engine then produces additional torque as required.
As compared to turbochargers as conventionally employed, which are constantly active, the turbocharger according to the present invention is used only when needed, that is, as noted, only when torque in excess of that available from the engine when in xe2x80x9cnormally-aspiratedxe2x80x9d mode is required. This allows both the engine and turbocharger to be designed to meet relatively well-defined objectives, providing further efficiency in use of fuel. Furthermore, because according to the invention the turbocharger is employed in a hybrid vehicle having one or more electric motors available to provide additional torque substantially immediately upon demand, the vehicle overall does not exhibit the slow response time of conventional turbocharged vehicles.
In a further refinement, the amount of time during which the motors will be used to supply torque in excess of that available from the engine in normally-aspirated mode before the turbocharger is activated is controlled responsive to the state of charge of the vehicle battery bank. More specifically, in general, when power in excess of the engine""s normally-aspirated capacity is required for a shorter period of time, e.g., when passing, at least the traction motor, or both of the electric motors, are energized by power from the battery. However, it is also within the scope of the invention to employ the turbocharger under circumstances calling for maximum acceleration, or when the state of charge of the battery is such that the electric motors will not be adequate to supply the torque required.
In addition to the operational advantages noted, provision of an engine having a xe2x80x9cturbocharger-on-demandxe2x80x9d in a hybrid vehicle allows the engine to be smaller than otherwise, that is, to provide adequate highway performance in a vehicle of a given weight. As the starting motor/generator must be sized such that when it is driven by the engine to charge the batteries (e.g., in extended city driving) the engine is loaded adequately to be operated efficiently, employment of a smaller engine allows use of a smaller starting motor/generator. For similar reasons, provision of a smaller engine allows it to be used to efficiently propel the vehicle in highway driving commencing at lower average speeds, resulting in turn in better fuel economy.